Our rabbit cages are in an unheated out-building and the bunnies do just fine here in rural Ontario (where night-time temperatures regularly dip below -20C). They should be okay as long as they're protected from draughts and their water's replenished often (they'll lick ice in a pinch, but it drops their core temperature, which isn't conducive to them gaining weight from their feed).
A hot water bottle won't hurt, but unless your equal to the task of replacing it every hour or so, it's probably best not to get them used to it.